A fuel cell is an electrochemical device that continuously produces electrical energy from a fuel (e.g. hydrogen) and an oxidant (e.g. O.sub.2) supplied continuously from external sources. One such H.sub.2 --O.sub.2 fuel cell, for example, is the so-called proton exchange membrane (PEM) fuel cell which uses an ion exchange membrane as the electrolyte. Hydrogen for the anode of such fuel cells may be provided from hydrogen storage tanks, or generated from dissociated methanol, gasoline, hydrazine or the like while air is used as the oxidant on the cathode side of the cell.
In addition to the fuel cell itself, fuel cell systems require a variety of auxiliary equipment (e.g. pumps, heat exchangers, fuel processors, combustors, water separators, etc.) to support the operation of the fuel cell. One such piece of auxiliary equipment is an air compressor for supplying compressed air to the cathode side of the cell, and to other of the auxiliary equipment, as needed. Fuel cell system compressors may be of the so-called "dynamic" type such as centrifugal or turbine compressors that have rapidly rotating rotor(s) that increase the velocity and pressure of the gas moving therethrough. The fuel cell compressor may also be of the so-called "positive displacement" type that has one or more rotor(s) in close proximity to each other, or to a stator. Positive displacement compressors are well known in the art and include rotary machines such as scroll machines, vane machines and screw machines, roots blowers, among others, and are generally characterized by an arrangement of members connected and constructed so that they (1) define and fill a cavity which is formed at the inlet port, (2) trap gas in the cavity, (3) transport the gas in the cavity toward a discharge port, with or without compression enroute, and (4) expel the gas from the cavity to the outlet port by mechanical displacement.
Positive-displacement air compressors can take many forms, but generally fall into two main classes, i.e. "wet" and "dry". "Wet" compressors, by design, have rotor(s) that engage each other, or a stator, across a film of lubricant (e.g. oil, water etc.) that is typically provided from a reservoir within the compressor. The lubricant prevents wear of the rotor(s)/stator and provides a liquid seal where the rotor(s) confront each other or a stator. The liquid seal retards backflow of the compressed gas into the compressor (i.e. reduces internal leakage). "Dry" compressors, on the other hand, by design, have rotor(s) which is/are closely spaced from each other, or from a stator, and have no sealing lubricant film therebetween. Rather, there is only a close clearance between the relatively moving parts, which clearance is typically maintained by timing gears or the like.
At low gas flow rates, wet compressors are generally more efficient than dry compressors, because of the moving liquid seal and the cooling effects provided by the lubricant. However, running a "wet" compressor dry (i.e. without the lubricant) would destroy it. Dry compressors, on the other hand, can run without lubricating the rotor(s), and are generally preferred for automobile-type fuel cell systems because they (1) require less input energy at their optimum design point than a wet compressor having the same capacity, (2) don't contaminate the oxidant gas, and (3) are not susceptible to freezing in cold weather applications. Moreover, dry compressors are suitable for high temperature operations, and for quick warm-up of a fuel cell system that has been allowed to stand idle and cool down. However, dry compressors work efficiently only at high gas flow rates, and high rotor speeds, which minimize internal leakage through the clearance spaces between the co-acting relatively moving parts (i.e. rotors and stators). At low gas flow rates (e.g. when the electrical demands on the fuel cell are low), dry compressors are quite inefficient because internal leakage becomes an increasingly larger percentage of the total flow through the compressor. An inefficient compressor, in turn, demands more fuel cell energy than an efficient compressor. In this regard, fuel cell system compressors are driven by electric motors that are energized by electric current withdrawn from the fuel cell. As such, the compressor drive motors are parasitic loads on the fuel cell system, in that the electrical current that they require must be subtracted from the current produced by the fuel cell which would otherwise be available to produce useful work (e.g. propel an automobile). Hence, the more inefficient the compressor, the greater the parasitic load that is placed on the fuel cell.